<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<style>
    .father{
        border: 5px solid;
        width: 400px;
        display: flow-root;
    }
    .son {
        background: pink;
        width: 400px;
        height: 50px;
        float: left;
    }
</style>

<body>
<div class="father">
    <div class="son"></div>
</div>
</body>
</body>
</html>
